Some of San Diego’s biggest names in life science companies are vanishing. And while the region has endured such events before, the shifts are sudden and immense.

Amylin Pharmaceuticals and Gen-Probe disappeared as independent companies in 2012, and Amylin is shutting down its San Diego operations. Carlsbad-based Life Technologies, the county’s biggest remaining life science company, announced less than a month ago that it would be purchased for $13.6 billion.

Investors in these companies have reasons to cheer, but some local biotech and medical device employees may lose out as the companies shed jobs.

San Diego has been through this many times, starting with the sale of its first biotech, Hybritech, in 1986. And, as before, new companies are growing up to replace them. Their growth shows the dynamism of local research and innovation that attracts investment.

San Diego County is now home to nine publicly traded life science companies with a market value of about $1 billion or more. They are Illumina, CareFusion, ResMed, Isis Pharmaceuticals, Arena Pharmaceuticals, DexCom, Santarus, Volcano and Acadia Pharmaceuticals.

Notably, three of these companies, Isis, Santarus and Arena, have moved into the $1 billion-plus rank during the last year. Shares of all three companies have roughly tripled in price since last spring. Isis is valued at $2.2 billion, Arena at $1.6 billion and Santarus at $1.2 billion.

But while San Diego excels in growing new life science companies, it lacks the stability of a hugely successful company too big for an easy acquisition, said John DeWitt, a vice president in the San Diego office of Lee Hecht Harrison. The company specializes in helping employees find new jobs.

In other words, San Diego lacks a biotech equivalent of the wireless communications giant Qualcomm, DeWitt said. And that makes it hard for displaced workers at the larger biotechs to find jobs.

“We’ve been astounded at how rapidly people have been picked up,” DeWitt said of laid-off biotech workers. “Unfortunately, an awful lot of them have to leave town. The (San Francisco) Bay Area is getting a lot of people. We’ve got a lot of people trying to figure out how to keep their house here or telecommute. The Bay Area has a lot of big, successful companies that are not for sale, and have money, and they’re pulling people.”

San Diego’s new top tier in life science companies may also end up as acquisitions. Some, like DNA sequencer Illumina, have fended off an unwanted takeover. Others haven’t had that experience. ResMed Chief Executive Mick Farrell said to his knowledge no formal takeover offer for the company has been made.

Farrell said ResMed is doing fine as an independent company, and intends to keep making innovative respiratory products to demonstrate it’s best off as its own entity.

CareFusion could be a takeover target by a private equity firm, according to a Bloomberg News story last month.

With that in mind, here’s the new list of San Diego’s largest, locally based biomedical companies, by market value.

Illumina, the DNA sequencing rival to Life Technologies, tops the list of biggest life science companies. The company fought off a hostile takeover bid last year by Swiss drug giant Roche. CEO Jay Flatley rejected Roche’s initial offer and a sweetened one as insufficient. Illumina is a favorite of biomedical researchers, who buy its sequencing equipment and then purchase consumables to perform the genetic analysis.

CareFusion, $7.6 billion

CareFusion, which places second, provides a variety of hospital equipment and products. Notable product lines include IVAC infusion pumps and Pxyis medicine dispensers, developed by the San Diego companies of those names. Their assets were sold and eventually purchased by Cardinal Health. In 2009, Cardinal spun off CareFusion. CareFusion shares reached a record high last month.

ResMed, $6.8 billion

ResMed makes devices to assist with breathing, including sleep apnea. CEO Mick Farrell took over this year from his father, Peter C. Farrell. Mick Farrell has been with the company for 13 years. The great majority of ResMed’s product sales are for sleep apnea. Recently, Resmed has begun selling its first product to treat chronic obstructive pulmonary disease, or COPD, characterized by difficulty in getting enough oxygen.

Isis Pharmaceuticals, $2.3 billion

The company makes drugs using its gene-blocking “antisense” technology. Over the years, Isis has been distinguished by the prodigious size and number of deals it has made with drug companies to develop medicines based on antisense. It has also been distinguished by a lagging stock price. CEO Stanley T. Crooke said at a shareholder’s meeting last June that getting the stock price up was a top priority. He got his wish. Isis shares, which closed at $9.99 the day of the meeting, ended Tuesday at $21.89.

Arena Pharmaceuticals, $1.6 billion

Arena reached a long-awaited milestone last year when the U.S. Food and Drug Administration approved its first drug. Called Belviq, the drug is intended to help the obese and overweight trim the poundage. The drug will soon be on sale by Arena's marketing partner Eisai. On Tuesday, Arena said Belviq was given its rating for risk of abuse by the Drug Enforcement Administration. The Schedule 4 rating, representing a low risk of abuse, represents the last approval needed before sales can begin.

DexCom, $1.3 billion

DexCom is the result of fusing medical devices with wireless technology, an important trend in San Diego medical devices. DexCom makes continuous glucose monitoring products to help diabetics keep their blood sugar level in check. A sensor placed just under the skin tracks glucose levels, and the information is wirelessly transmitted to a readout device.

Santarus, $1.2 billion

A specialty pharmaceutical company, Santarus licenses and develops a variety of products for diseases treated by specialists. It sells Glumetza and Cycloset, for diabetes; Fenoglide for high cholesterol; Uceris for ulcerative colitis; and Zegerid for acid reflux. A patent suit victory for Zegerid and the launch of Uceris were recent highlights.

Volcano Corp., $999 million

Volcano makes medical instruments to help diagnose and treat cardiovascular disease. Products include many kinds of blood vessel catheters to assist in imaging, and ultrasound devices to help place stents. The company relocated to San Diego from Rancho Cordova in Northern California in 2008.

Acadia Pharmaceuticals, $959 million

Acadia got approval last month to file a marketing application for its flagship drug, pimavanserin. The drug treats Parkinson’s disease psychosis. The consent by the U.S. Food and Drug Administration doesn’t mean the drug itself will be approved, but is a positive sign. The FDA agreed to let Acadia dispense with a second planned Phase 3 study.